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Local recurrent networks in neocortex are critical nodes for sensory processing, but their regulation by experience is much less understood than for long-distance (translaminar or cross-columnar) projections. We studied local L2/3 recurrent networks in rat somatosensory cortex during deprivation-induced whisker map plasticity, by expressing channelrhodopsin-2 (ChR2) in L2/3 pyramidal cells and measuring light-evoked synaptic currents in exvivo S1 slices. In columns with intact whiskers, brief light impulses evoked recurrent excitation and supralinear inhibition. Deprived columns showed modestly reduced excitation and profoundly reduced inhibition, providing a circuit locus for disinhibition of whisker-evoked responses observed in L2/3 invivo. Slower light ramps elicited sustained gamma frequency oscillations, which were nearly abolished in deprived columns. Reduction in gamma power was also observed in spontaneous LFP oscillations in L2/3 of deprived columns invivo. Thus, L2/3 recurrent networks are a powerful site for homeostatic modulation of excitation-inhibition balance and regulation of gamma oscillations.

Apical dendritic bundles from pyramidal neurons are a prominent feature of cortical neuropil but with significant area specializations. Here, we investigate mechanisms of bundle formation, focusing on layer (L) 2 bundles in rat granular retrosplenial cortex (GRS), a limbic area implicated in spatial memory. By using microarrays, we first searched for genes highly and specifically expressed in GRS L2 at postnatal day (P) 3 versus GRS L2 at P12 (respectively, before and after bundle formation), versus GRS L5 (at P3), and versus L2 in barrel field cortex (BF) (at P3). Several genes, including neurotrophin-3 (NT-3), were identified as transiently and specifically expressed in GRS L2. Three of these were cloned and confirmed by in situ hybridization. To test that NT-3-mediated events are causally involved in bundle formation, we used in utero electroporation to overexpress NT-3 in other cortical areas. This produced prominent bundles of dendrites originating from L2 neurons in BF, where L2 bundles are normally absent. Intracellular biocytin fills, after physiological recording in vitro, revealed increased dendritic branching in L1 of BF. The controlled ectopic induction of dendritic bundles identifies a new role for NT-3 and a new in vivo model for investigating dendritic bundles and their formation.

The amygdala complex participates in multiple systems having to do with affective processes. It has been implicated in human disorders of social and emotional behavior, such as autism. Of the interconnected functional networks, considerable research in rodents and primates has focused on connections between the amygdala and orbitofrontal cortex (OFC). The amygdala projects to OFC by both a direct amygdalocortical (AC) pathway and an indirect pathway through mediodorsal thalamus. In the rat, retrograde tracer experiments indicate that the AC and amygdalothalamic (AT) pathways originate from separate populations, and may therefore convey distinctive information, although the characteristics of these pathways remain unclear. To investigate this issue in monkeys we made anterograde tracer injections in the basolateral amygdala complex (BLC; n = 3). Three distinctive features were found preferentially associated with the AT or AC pathways. First, AT terminations are large (average diameter = 3.5 microm; range = 1.2-7.0 microm) and cluster around proximal dendrites, in contrast with small-bouton AC terminations. Second, AT terminations form small arbors (diameter approximately 0.1 mm), while AC are widely divergent (often >1.0 mm long). The AT terminations features are reminiscent of large bouton, "driver" corticothalamic terminations. Finally, AC but not AT terminations are positive for zinc (Zn), a neuromodulator associated with synaptic plasticity. From these results we suggest that AC and AT terminations originate from distinct populations in monkey as well as in rodent. Further work is necessary to determine the degree and manner of their segregation and how these subsystems interact within a broader connectivity network.

The claustrum is a phylogenetically conserved structure, with extensive reciprocal connections with cortical regions, and has thus been considered important for sensory, motor, emotional, and mnemonic coordination or integration. Here, we show by in situ hybridization that the adult monkey claustrum is strongly positive for NETRIN-G2, a gene encoding a glycosyl phosphatidyl-inositol-linked membrane protein, which constitutes a subfamily with NETRIN-G1 within the netrin/UNC6 family. There is a conspicuous dorsal/ventral differentiation, where the label is stronger in the ventral claustrum. NETRIN-G2 positive neurons are not GABAergic, but rather correspond to claustrocortical projection neurons, as demonstrated by retrograde transport of Fast Blue from cortical injections and by double in situ hybridization for NETRIN-G2 and GAD67. Since NETRIN-G2 is known to be preferentially expressed in cortex, in contrast with the thalamically expressed NETRIN-G1, these results raise the possibility of some functional similarity in regulation of excitatory neural transmission in the claustrum and cortex.

In Drosophila melanogaster, Slit acts as a repulsive cue for the growth cones of the commissural axons which express a receptor for Slit, Roundabout (Robo), thus preventing the commissural axons from crossing the midline multiple times. Experiments using explant culture have shown that vertebrate Slit homologues also act repulsively for growth cone navigation and neural migration, and promote branching and elongation of sensory axons. Here, we demonstrate that overexpression of Slit2 in vivo in transgenic zebrafish embryos severely affected the behavior of the commissural reticulospinal neurons (Mauthner neurons), promoted branching of the peripheral axons of the trigeminal sensory ganglion neurons, and induced defasciculation of the medial longitudinal fascicles. In addition, Slit2 overexpression caused defasciculation and deflection of the central axons of the trigeminal sensory ganglion neurons from the hindbrain entry point. The central projection was restored by either functional repression or mutation of Robo2, supporting its role as a receptor mediating the Slit signaling in vertebrate neurons. Furthermore, we demonstrated that Islet-2, a LIM/homeodomain-type transcription factor, is essential for Slit2 to induce axonal branching of the trigeminal sensory ganglion neurons, suggesting that factors functioning downstream of Islet-2 are essential for mediating the Slit signaling for promotion of axonal branching.

Rodent whisker sensation occurs both actively, as whiskers move rhythmically across objects, and in a passive mode in which externally applied deflections are sensed by static, non-moving whiskers. Passive whisker stimuli are robustly encoded in the somatosensory (S1) cortex, and provide a potentially powerful means of studying cortical processing. However, whether S1 contributes to passive sensation is debated. We developed 2 new behavioral tasks to assay passive whisker sensation in freely moving rats: Detection of unilateral whisker deflections and discrimination of right versus left whisker deflections. Stimuli were simple, simultaneous multi-whisker deflections. Local muscimol inactivation of S1 reversibly and robustly abolished sensory performance on these tasks. Thus, S1 is required for the detection and discrimination of simple stimuli by passive whiskers, in addition to its known role in active whisker sensation.

The habenulae are part of an evolutionarily highly conserved limbic-system conduction pathway that connects telencephalic nuclei to the interpeduncular nucleus (IPN) of the midbrain [1]. In zebrafish, unilateral activation of the Nodal signaling pathway in the left brain specifies the laterality of the asymmetry of habenular size [2–5]. We show “laterotopy” in the habenulo-interpeduncular projection in zebrafish, i.e., the stereotypic, topographic projection of left-sided habenular axons to the dorsal region of the IPN and of right-sided habenular axons to the ventral IPN. This asymmetric projection is accounted for by a prominent left-right (LR) difference in the size ratio of the medial and lateral habenular sub-nuclei, each of which specifically projects either to ventral or dorsal IPN targets. Asymmetric Nodal signaling directs the orientation of laterotopy but is dispensable for the establishment of laterotopy itself. Our results reveal a mechanism by which information distributed between left and right sides of the brain can be transmitted bilaterally without loss of LR coding, which may play a crucial role in functional lateralization of the vertebrate brain [6, 7].

In the cerebral cortex, the voltage-gated potassium channel, Kv3.1b, a splicing variant of Kv3.1, has been associated with fast-firing interneurons. Here, we report strong expression of Kv3.1b-protein and mRNA in both Betz and Meynert pyramidal cells of the monkey, as shown by immunohistochemistry and in situ hybridization. Strong expression also occurs in large pyramidal neurons in layer 5 of several cortical areas. In addition, most of these Betz and layer 5 pyramids, and about 10% of the labeled Meynert cells weakly co-expressed the calcium binding protein parvalbumin. Electron microscopy shows that the expression of Kv3.1b is localized to the somal and proximal dendritic cytoplasmic membrane, as expected for a channel protein. These results suggest that some large pyramidal neurons may constitute a functional subpopulation, with a distinctive distribution of voltage-gated potassium channels capable of influencing their repetitive firing properties.

The Brx1 homeobox gene has been isolated and shown to be expressed in the zona limitans intrathalamica (ZLI) of the mouse embryo. Brx1 is a member of the Brx gene family and comprises the genes for Brx1a and Brx1b, which differ in the sequence in the region located on the 5'-terminal side of the homeobox. The complete amino acid sequences of the open reading frame of Brx1a and Brx1b were determined and each was found to be similar to that of Rgs, the mouse homologue of the Rieger syndrome associated human RIEG gene (RGS), to the extent that the sequence of Rgs has been clarified. Brx1 was strongly expressed in the mammillary area as well as in the ZLI of the mouse embryonic brain. Homologues of Brx1a and Brx1b were isolated in chick in which the expression of Brx1 in the ventral diencephalon was well conserved. The expression of Brx1 along with that of Sonic hedgehog (Shh), Nkx2.2, Dlx1 and Arx was examined at the time of the formation of ZLI in mouse embryos. The expression of Shh was initially noted in the ventricular zone of the presumptive ZLI and was then replaced by that of Brx1 at the time of radial migration of the neuroepithelial cells. Nkx2.2 was widely expressed in the ventricular zone of presumptive ZLI and also as a narrow band in the mantle zone. The expression of Dlx1 and Arx in the presumptive ventral thalamus extended as far as ZLI and overlapped with that of Brx1. The Dlx1- and Arx-expressing cells in ZLI, which extended towards the lateral (pial) surface of the diencephalic wall, differed from those expressing Nkx2.2 and Brx1. The embryonic ventral lateral geniculate nucleus present in the visual pathway was eventually formed from these cells. Each homeobox gene was also expressed regionally in the nucleus, suggesting that the nucleus is comprised of subdivisions.

A histopathological study was made on an infection of Streptococcus sp. in cultured tilapia, Sarotherodon niloticus, which occurred in warm water ponds during the winter of 1980. The external signs were dermal hemorrhage and exophthalmia. The internal signs were dropsy, epicarditis, peritonitis, pale-colored liver, splenomegary and nodule formation in gonads. On histopathological examination, the bacterial dissemination was systemic. Abscesses and granulomas were found to develope in the infected orbital adipose tissue of the exophthalmic eyes. Infiltration of bacteria-laden macrophages and granuloma formation were observed in the infected lesions of the epicardium, capsules of the liver and spleen, peritoneum, stomach, intestine, brain, ovary and testis.

A histopathological study was made on Pseudomonas fluorescens infection which occurred in the spring of 1979 in pond-cultured tilapia, Sarotherodon niloticus. The infected fish showed exophthalmia, dark body coloration, spotty or nodular lesions in the liver, spleen, kidney and gills, and inflamed swim-bladder. The bacterial dissemination was systemic. The histopathological aspects were abscess formation in eyes, spleen and swim-bladder, and focal necrosis in the liver, gills and kidney of some diseased fish. The other fish showed granuloma formation in all infected lesions.

Slit is expressed in the midline of the central nervous system both in vertebrates and invertebrates. In Drosophila, it is the midline repellent acting as a ligand for the Roundabout (Robo) protein, the repulsive receptor which is expressed on the growth cones of the commissural neurons. We have isolated cDNA fragments of the zebrafish slit2 and slit3 homologues and found that both genes start to be expressed by the midgastrula stage well before the axonogenesis begins in the nervous system, both in the axial mesoderm, and slit2 in the anterior margin of the neural plate and slit3 in the polster at the anterior end of the prechordal mesoderm. Later, expression of slit2 mRNA is detected mainly in midline structures such as the floor plate cells and the hypochord, and in the anterior margins of the neural plates in the zebrafish embryo, while slit3 expression is observed in the anterior margin of the prechordal plate, the floorplate cells in the hindbrain, and the motor neurons both in the hindbrain and the spinal cord. To study the role of Slit in early embryos, we overexpressed Slit2 in the whole embryos either by injection of its mRNA into one-cell stage embryos or by heat-shock treatment of the transgenic embryos which carries the slit2 gene under control of the heat-shock promoter. Overexpression of Slit2 in such ways impaired the convergent extension movement of the mesoderm and the rostral migration of the cells in the dorsal diencephalon and resulted in cyclopia. Our results shed light on a novel aspect of Slit function as a regulatory factor of mesodermal cell movement during gastrulation.

Background The specificity of synaptic connections is fundamental for proper neural circuit function. Specific neuronal connections that underlie information processing in the sensory cortex are initially established without sensory experiences to a considerable extent, and then the connections are individually refined through sensory experiences. Excitatory neurons arising from the same single progenitor cell are preferentially connected in the postnatal cortex, suggesting that cell lineage contributes to the initial wiring of neurons. However, the postnatal developmental process of lineage-dependent connection specificity is not known, nor how clonal neurons, which are derived from the same neural stem cell, are stamped with the identity of their common neural stem cell and guided to form synaptic connections. Results We show that cortical excitatory neurons that arise from the same neural stem cell and reside within the same layer preferentially establish reciprocal synaptic connections in the mouse barrel cortex. We observed a transient increase in synaptic connections between clonal but not nonclonal neuron pairs during postnatal development, followed by selective stabilization of the reciprocal connections between clonal neuron pairs. Furthermore, we demonstrate that selective stabilization of the reciprocal connections between clonal neuron pairs is impaired by the deficiency of DNA methyltransferase 3b (Dnmt3b), which determines DNA-methylation patterns of genes in stem cells during early corticogenesis. Dnmt3b regulates the postnatal expression of clustered protocadherin (cPcdh) isoforms, a family of adhesion molecules. We found that cPcdh deficiency in clonal neuron pairs impairs the whole process of the formation and stabilization of connections to establish lineage-specific connection reciprocity. Conclusions Our results demonstrate that local, reciprocal neural connections are selectively formed and retained between clonal neurons in layer 4 of the barrel cortex during postnatal development, and that Dnmt3b and cPcdhs are required for the establishment of lineage-specific reciprocal connections. These findings indicate that lineage-specific connection reciprocity is predetermined by Dnmt3b during embryonic development, and that the cPcdhs contribute to postnatal cortical neuron identification to guide lineage-dependent synaptic connections in the neocortex. Electronic supplementary material The online version of this article (doi:10.1186/s12915-016-0326-6) contains supplementary material, which is available to authorized users.